As a next-generation treatment, gene therapy for treating diseases by controlling gene expression has been greatly anticipated. The biggest problem with gene therapy is that introduction efficiency at the time when genes are introduced into target cells or tissues is insufficient. Particularly, in order to realize gene therapy through systemic administration, it is necessary that genes be stably circulated in the blood and accumulated on target tissues and that gene expression be effectively performed after genes have entered target tissues. Here, in order to solve these problems, development of gene carriers having better introduction efficiency to target cells or the like, and gene expression efficiency in target cells has been actively promoted.
For example, it is known that a polymer in which a primary structure is precisely controlled is spontaneously organized and may form a higher-order structure such as a micelle or a vesicle and use of a structure obtained by a polymer being self-organized in such a manner has been previously examined in various fields including drug delivery systems and material science. For example, PTL 1 discloses an electrostatic binding type polymer micelle drug carrier formed of a block copolymer including an uncharged segment (uncharged polymer chain block) and a charged segment (charged polymer chain block) and capable of encapsulating a drug having an opposite charge to that of the charged segment, in a core portion. When a cationic segment is used as the charged segment, it is possible to encapsulate DNA in the core portion.
Furthermore, research performed for stabilization of a polymer micelle in various manners has been reported. For example, in regard to an electrostatic binding type polymer micelle drug carrier. PTL 2 discloses an electrostatic binding type polymer micelle drug carrier stabilized by crosslinking block copolymers through a crosslinking agent. In addition, PTL 3 discloses a block copolymer formed by containing an uncharged hydrophilic polymer chain block and a cationic polyamino acid chain block in which a hydrophobic group is introduced into a part of the side chain thereof. By virtue of a hydrophobic group introduced into the side chain of the block copolymer, interfacial energy is increased, thereby the cohesive force in a micelle becomes higher and the core becomes smaller, and thus, the polymer micelle is stabilized.